A popping question

[mdh]

Could someone help me understand one aspect of an exercise in K&R II;
(page 76)

Given function:

double pop( void){

....return a double from a stack if present

}

how does this work?

/* adds 2 doubles*/

case '+' :

push ( pop() + pop()) ;

I would have thought you would have to do something like

double firstpop, secondpop;

firstpop=pop();
secondpop=pop() ;
push ( firstpop+second pop);

ie where do the doubles go if they are not assigned. It obviously goes
somewhere and it does work, it's just that I do not see how.

Thanks in advance.

[Ben Pfaff]

"mdh" <md**@comcast.n etwrites:

how does this work?

....

push ( pop() + pop()) ;

I would have thought you would have to do something like

double firstpop, secondpop;

firstpop=pop();
secondpop=pop() ;
push ( firstpop+second pop);

ie where do the doubles go if they are not assigned. It obviously goes
somewhere and it does work, it's just that I do not see how.

It doesn't matter. It's the compiler's job to reserve space for
temporaries used in expressions. The programmer doesn't have to
worry about it.
--
"Programmer s have the right to be ignorant of many details of your code
and still make reasonable changes."
--Kernighan and Plauger, _Software Tools_

[mdh]

"mdh" <md**@comcast.n etwrites:

ie where do the doubles go if they are not assigned. It obviously goes
somewhere and it does work, it's just that I do not see how.

Ben Pfaff wrote:

It doesn't matter. It's the compiler's job to reserve space for
temporaries used in expressions. The programmer doesn't have to
worry about it.

So is it fair to assume that "pop" is in a sense replaced by it's
value, and any assignment is "additonal" but not necessary to the
execution of "pop"?

[Ben Pfaff]

"mdh" <md**@comcast.n etwrites:

>"mdh" <md**@comcast.n etwrites:

ie where do the doubles go if they are not assigned. It obviously goes
somewhere and it does work, it's just that I do not see how.

Ben Pfaff wrote:

>It doesn't matter. It's the compiler's job to reserve space for
temporaries used in expressions. The programmer doesn't have to
worry about it.

So is it fair to assume that "pop" is in a sense replaced by it's
value, and any assignment is "additonal" but not necessary to the
execution of "pop"?

Do you have an idea that the result of a function must be
immediately assigned to a variable, or that otherwise it is lost?
That is how I interpret your words. This idea is wrong. All of
the following are allowed in C, given an appropriate function foo
and variable x:
foo(); /* Throws away return value. */
x = foo(); /* Assigns return value. */
x = foo() + 1; /* Assigns return value plus 1. */
x = foo() + foo(); /* Calls foo twice, assigns the sum. */
--
int main(void){char p[]="ABCDEFGHIJKLM NOPQRSTUVWXYZab cdefghijklmnopq rstuvwxyz.\
\n",*q="kl BIcNBFr.NKEzjwC IxNJC";int i=sizeof p/2;char *strchr();int putchar(\
);while(*q){i+= strchr(p,*q++)-p;if(i>=(int)si zeof p)i-=sizeof p-1;putchar(p[i]\
);}return 0;}

[mdh]

Ben Pfaff wrote:
Do you have an idea that the result of a function must be

immediately assigned to a variable, or that otherwise it is lost?
That is how I interpret your words. This idea is wrong.

no....I did not have an fixed idea...although it may have come across
that way...but your example clarifies it for me.

thanks.

[CBFalconer]

Ben Pfaff wrote:

"mdh" <md**@comcast.n etwrites:

>how does this work?

...

>push ( pop() + pop()) ;

I would have thought you would have to do something like

double firstpop, secondpop;

firstpop=pop() ;
secondpop=pop( );
push ( firstpop+second pop);

ie where do the doubles go if they are not assigned. It obviously goes
somewhere and it does work, it's just that I do not see how.

It doesn't matter. It's the compiler's job to reserve space for
temporaries used in expressions. The programmer doesn't have to
worry about it.

Fine for + and *. Change it to - or / and things are different.

--
Some informative links:
news:news.annou nce.newusers
http://www.geocities.com/nnqweb/
http://www.catb.org/~esr/faqs/smart-questions.html
http://www.caliburn.nl/topposting.html
http://www.netmeister.org/news/learn2quote.html

--
Posted via a free Usenet account from http://www.teranews.com

[Ben Pfaff]

CBFalconer <cb********@yah oo.comwrites:

Ben Pfaff wrote:

>"mdh" <md**@comcast.n etwrites:

>>how does this work?

...

>>push ( pop() + pop()) ;

I would have thought you would have to do something like

double firstpop, secondpop;

firstpop=pop( );
secondpop=pop ();
push ( firstpop+second pop);

ie where do the doubles go if they are not assigned. It obviously goes
somewhere and it does work, it's just that I do not see how.

It doesn't matter. It's the compiler's job to reserve space for
temporaries used in expressions. The programmer doesn't have to
worry about it.

Fine for + and *. Change it to - or / and things are different.

The OP's question was about memory. You are talking about order
of evaluation. The K&R2 page in question mentions that - and /
will not (reliably) do what we want for order of evaluation, so
there was no reason for me or the OP to bring it up.
--
Just another C hacker.

[mdh]

Ben Pfaff wrote:

>
The OP's question was about memory. You are talking about order
of evaluation. The K&R2 page in question mentions that - and /
will not (reliably) do what we want for order of evaluation, so
there was no reason for me or the OP to bring it up.

Hi Ben,
Correct....I probably did think that any value had to be assigned hence
the question. I think, even in C, one has to take certain things as
"given" as your example clarified. And yes...I am aware of the order of
things.

[Barry Schwarz]

On 21 Sep 2006 15:36:21 -0700, "mdh" <md**@comcast.n etwrote:

>
Ben Pfaff wrote:

>>
The OP's question was about memory. You are talking about order
of evaluation. The K&R2 page in question mentions that - and /
will not (reliably) do what we want for order of evaluation, so
there was no reason for me or the OP to bring it up.

Hi Ben,
Correct....I probably did think that any value had to be assigned hence
the question. I think, even in C, one has to take certain things as
"given" as your example clarified. And yes...I am aware of the order of
things.

I hope you meant to say "the lack of order".
Remove del for email

[mdh]

Ben Pfaff wrote:

>
The OP's question was about memory. You are talking about order
of evaluation. The K&R2 page in question mentions that - and /
will not (reliably) do what we want for order of evaluation, so
there was no reason for me or the OP to bring it up.

Hi Ben,
Correct....I probably did think that any value had to be assigned hence
the question. I think, even in C, one has to take certain things as
"given" as your example clarified. And yes...I am aware of the order of
things.

Barry Schwarz wrote:

I hope you meant to say "the lack of order".

Clearly carrying some deeper meaning, deeper than my knowledge. Care
to explain?